The present invention relates to a process for the selective reclamation treatment of used foundry sands for reuse in foundry sand mold in place of new sand.
Iron and steel foundry molding technology uses resources which depend greatly on the requirements and constraints of environmental compatibility. This is particularly the case with molding sand which determines to a great extent the moldability of a casting mold.
A differentiation is made in the technology between natural sand and synthetic sands. The latter are pure, usually washed and classified quartz sands without any additions of organic and/or inorganic additives or impurities.
The pure sand described above, with its grain composition determined by the size of the individual grains, has to be made moldable by additions of precisely determined and measured additives of an organic and inorganic nature.
In the normal molding sand cycle of an iron and steel foundry, that is with clay-bonded wet cast sand, the greatest portion of the used sand collected at the emptying point of a casting is conveyed via a preparation plant for re-use in the wet cast molding operation. This used sand is a mixture of predominantly clay-bonded molding sand and smaller amounts of chemically bonded core sand which was first introduced into the cycle via the core molding as new sand. Still active bonding clay (bentonite) and carbonaceous residues, especially carbonized porous coal dust, is regularly contained in the used sand. In addition, the sand grains are subject to increasing structural change with repeated circulation as part of the bonding clay is dead-burned by the effect of the heat of the cast metal and remains stuck to the quartz grains as a ceramic porous surface layer (so-called oolithisation the art).
Process technologies have been developed and used which separate the used additives, such as bentonite and carbonaceous residues, from the quartz grains so as to convey at least the quartz sand, collected in large amounts, for further re-use. This separation of quartz sand and used binding agent etc. has varied success in these processes especially with respect to the residues still surrounding the individual grains after separation. The total percentage amount of these residuals is determined by trial methods know in the art. The determined parameters are combined under the headings annealing loss, slurry, sieve analysis and pH value determination, and determine in their entirety the further re-use of the sand.
The sand quality parameters employed represent a total value of a specific sand test quantity. In practice, however, the sand as it occurs for reclamation is assessed in its entirety without reference to its grain size composition.
Thus, in the prior art the quality assessments of a sand, that is a measurement of these parameters, do not take into account the size of the individual grains but only the resulting average value of the test sand quantity. However, present mixing processes and apparatus belonging thereto make possible an extremely uniform and homogenous distribution of binding materials to the surfaces of the sand grains. This means that the addition of binding material is related to weight and not surface area. The proportional surfaces of a test quantity of sand means that the binding material dosing does not relate to the number of small or larger grains but rather to the weight quantity.
However, in practice gravimetrically similar test amounts of sand, separated according to larger and smaller grains, have completely different surfaces.
If one proceeds from the fact that the surface charging with additives of larger and smaller grains is the same, this means that a gram of large grains, has in total less binding material than the same comparative amount of small grains because of the larger surface ratios.
In the prior art, totality of the occurring sand to be reclaimed in its entirety, regardless of which system technology it is treated with, undergoes a blanket cleaning process with respect to time and energy application. A differentiating factor with regard to time, system technology and hence energy application should be called for.
Accordingly, it is the principle object of the present invention to provide a process for the reclamation of used foundry sand in which treatment of the sand is carried out selectively based on the differences in grain size of the sand. In accordance with the present invention, a uniform degree of purification of the quartz sand grains can be achieved irrespective of their grain size.